The present invention relates generally to a carbonaceous material (hereinafter referred to as "carbon material") and a composite product obtained therefrom, and particularly relates to the composite product comprising the carbon materials joined by the melt-adhesion of a tetrafluoroethylene resin or a tetrafluoroethylene resin mixed with a highly electroconductive carbon black interposed between the carbon materials, and the process for producing the composite carbon product.
Furthermore the present invention relates to a composite electrode substrate for a fuel cell of phosphoric acid type comprising the above-mentioned composite carbon product, wherein one of the two carbon materials is the extended part of the separator beyond the electrode of the above-mentioned composite electrode substrate and the other of the two carbon materials is a peripheral sealer, the peripheral sealer and a gas-distributor or a gas manifold, each of them being contact with the periphery of the electrode.
In recent years, carbonaceous products using carbonaceous materials such as carbon fiber, carbon particles as carbon aggregate, etc. have been used in various industrial fields, and higher requirements for the improvements of the productivity, the physical properties, etc. of the product have been largely raised with the technical progress and the increase of the demand for the carbonaceous products.
Although the carbon material is excellent in the physical properties as a raw material, for instance, heat-resistance, resistance to chemicals, electroconductivity, mechanical strength, etc., a development of carbonaceous composite materials which have been produced by joining the carbon materials of the same or different qualities in combination has been promoted for further making the most of these excellent physical properties. Such a carbonaceous composite material has been used hitherto as a composite product obtained by simply adhering the carbon materials by using an adhesive, however, there have been problems in resistance to chemicals, electroconductivity, dimensional stability, etc. thereof.
In recent years, a method for solving the above-mentioned problems has been conceived, wherein the carbon materials are joined together by using an adhesive and the thus composed materials are calcined to be one body.
However, in the case of producing the composite product by such a process, there are occasions where the carbon materials exfoliate on the joining surface thereof and cracks are caused in the product due to the difference of expansion and contraction rate between the carbon materials during the step of calcination, thus resulting in the reduction of the yield of production.
In addition, there is a high requirement for the development and utilization of a fuel cell and its circumferential system as the apparatus for generating a clean energy or a freely make and breakable electrogenerating apparatus which can contribute to the resource-economization by the levelling of the operation in steam-power generation or water-power generation or the improvement of the energy efficiency.
Hitherto, as fuel cells, the fuel cell of the bipolar type in which the bipolar separator obtained by rib-processing the gas-impermeable thin plate made of graphite is used after combining with a porous carbon flat plate has been publicly known, however, as compared with such a fuel cell, a development of a fuel cell of the monopolar type composed of stacking a porous electrode substrate.having ribs on one side thereof and the other flat side, a catalyst layer, a matrix impregnated with an electrolyte and a separator has been carried out. In such a fuel cell of the monopolar type, the reactant gas (oxygen or hydrogen) diffuses from the gas-flow channel formed by the ribs disposed on the electrode substrate to the flat surface of the electrode.
Although such an electrode substrate is made generally of the carbon material from the viewpoint of the physical properties such as heat-resistance, corrosion-resistance, electroconductivity, mechanical strength and the easiness of retaining porosity, etc. and is used after stacking as shown above, it is difficult to bring the flatness of the flat top part of the ribs to perfection and the electric and thermal contact resistance between the separator and the rib becomes too large to be not neglected. Generally, it has been generally said that the above-mentioned contact resistance is larger than the transmission resistance within the substrate by several times and causes the decisive demerits of the unevennes of the temperature distribution between the cells and the reduction of the generation efficiency.
As an object of solving such a problem of contact resistance, a composite electrode substrate produced by adhering the electrode substrate to the separator, etc. in the stack construction of the above-mentioned fuel cell and further calcining the thus adhered materials into one body as carbon has been proposed. Although in such a composite electrode substrate, the contact resistance which was present on the contact surfaces can be made zero by joining into one body, there are occasions, as has been stated, of exfoliation of the adhered surface of the carbon materials and of causing the cracks in the product due to the difference of expansion and contraction rates between the carbon material and the adhesive, because the electrode substrate is produced by joining the carbon materials together and further carbonizing and calcining the thus composed materials. Such occasions result in the reduction of the productive efficiency, and accordingly, the improvement thereof has been demanded.
As has been described above, to the carbonaceous composite material, very severe requirements of (1) not causing exfoliation of the carbon material therefrom at the time of producing and using the electrode substrate, (2) not causing the cracks in the product and also, (3) retaining the excellent specificities originally possessed by the carbon mateial, for instance, heat-resistance, resistance to chemicals, mechanical strength and electric specificities have been imposed, and therefore, the production of the carbonaceous composite material accompanies by an extraordinary difficulty.
As a result of the present inventors' studies on the process for joining the carbon materials together in the production of the carbonaceous composite material, it has been found by the present inventors that the carbonaceous composite material having desirable specifrcities can be produced with a favorable productivity by interposing a flexible graphite sheet between the carbon materials wherein the thus interposed graphite sheet acts as the buffer layer of the difference of the expansion and contraction rates of the carbon materials in the step of calcination. On the basis of their above-mentioned finding, the present inventors have filed a patent application (refer to U.S. patent application Ser. No. 812,724).
Further, as a result of the present inventors' studies from the conception that the difference of the expansion and contraction rates between the porous carbonaceous layer and the gas-impermeable layer (the separator) may be reduced or removed by a buffer layer interposed between the porous carbonaceous layer and the gas-impermeable layer (the separator) in the view point that the exfoliation occurring in the composite electrode substrate for a fuel cell in the step of calcination (up to the highest temperature of 3000.degree. C.) is due to the difference of the thermal expansion rate between the porous carbonaceous layer and the gas-impermeable layer (the separator) in the temperature-raising step or to the difference of the thermal contraction rate between the above-mentioned two layers in the cooling step to room temperature after completing the calcination, it has been found by the present inventors that the inter-layer exfoliation which has been a problem can be improved by inserting a flexible carbon sheet, as a material for a buffer layer, which is relatively large in the rate of thermal expansion and contraction, has an adhesion to an adhesive, etc. and is not so permeable to gases, between the porous carbonaceous layer and the separator of the above-mentioned electrode substrate and joining the above-mentioned two layers via a carbonizable adhesive.
However, the substrate as the electrode in the fuel cell of phosphoric acid type in general is stacked so that one side thereof contacts to the matrix of phosphoric acid and the other side thereof faces to the separator.
In addition, on forming a fuel cell by stacking the electrode substrates, (1) a peripheral sealer or the peripheral sealer and a gas-distributor are disposed on the side of the electrode substrate parallel or parallel and perpendicular to the flow channels therein respectively to prevent the diffusion of the reactant gas from the side of the electrode to out side or (2) a manifold is disposed on the each side of the electrode for supplying the reactant gas to the fuel cell and at the same time, for preventing the diffusion of the reactant gas from the side of the electrode substrate to outside.
Accordingly, particularly in the case where the composite electrode substrate is formed of the porous and carbonaceous electrode up to the edge part thereof and the flow channels of the reactant gas opens directly at the edge part in the composite electrode substrate of the external manifold type, the peripheral sealer which is compact and carbonaceous and the electrode which is porous and carbonaceous are disposed opposite each other across the separator on the peripheral region of the separator, and there has been a problem of causing a certain degree of a warp or a strain in the joining part of the materials due to the difference of the thermal shrinkage between the materials even by the intervention of the flexible carbon sheet. As the means for preventing the above-mentioned warp, the materials with an extremely small difference of the thermal contraction rate should be selected, and such a restriction has been the obstruction in the production of the composite product.
In addition, as the other problems of the conventional electrode substrate for fuel cells, (1) there has been a possibility of causing exfoliation between the materials and leakage of the reactant gas through the joining part due to the poor resistance of the carbon cement used in joining the materials of the electrode substrate to phosphoric acid and (2) there has been a problem in the point of mechanical strength of the electrode substrate resulting in breaking on handling in the case where the area of the substrate is too large, because the electrode substrate is a thin plate.
Further, a method of joining the porous electroductive materials wherein the gas-impermeability between the porous electroconductive materials has been increased, has been proposed recently. According to the proposed method, the porous electroconductive material is impregnated with a fluorinated ethylene-propylene polymer, a polysulphone resin, etc., and the thus impregnated layer is joined as an interface to another electroconductive material by hot-ressing while maintaining electroconductivity through the gas impervious region (for instance, refer to U.S. Pat. No. 4,505,992).
However, in the case of using the above-mentioned methods, although the passage of the gas between the two carbon materials is prevented by the thus resin-impregnated carbon layer, since such a resin is low in melting viscosity, the usage of the thus obtained composite material impregnated with such a resin is limited.
As a result of the present inventors studies on the process which has overcome the above-mentioned defects of the conventional process and can join the carbon materials which have a large mechanical strength and can be used in an atmosphere at a temperature as high as about 350.degree. C., it has been found by the present inventors that a composite product comprising the carbon materials, which is excellent in heat-resistance and resistance to chemicals and has an improved buffer action to the thermal expansion and adhesive strength, is obtained by joining the carbon materials together with the melt-adhesion of a tetrafluoroethylene resin interposed between the two carbon materials, and at the same time, a composite product which comprises the carbon materials, is excellent in resistance to chemicals and also has an electroconductivity is obtained by mixing a highly electroconductive carbon black with the tetrafluoroethylene resin, and on the basis of their above-mentioned findings, the present inventors have attained the present invention.
Namely, the first object of the present invention is to provide a composite product which comprises the carbon materials and has excellent physical properties, particularly the improved properties such as heat-resistance, resistance to chemicals and adhesive strength.
Furthermore, the second object of the present invention is to provide a process for producing a composite product comprising the carbon materials having the excellent physical properties and not having the demerits of the conventional techniques.
Still more, the third object of the present invention is to provide an electroconductive composite product comprising the carbon materials and having excellent physical properties, particularly the improved properties of heat-resistance, resistance to chemicals and adhesive strength.
In addition, the fourth object of the present invention is to provide a process for producing a carbonaceous and electroconductive composite product which has excellent physical properties and not having the demerit of the conventional technique.
Furthermore, the fifth object of the present invention is to provide a composite electrode substrate for a fuel cells, wherein the compact and carbonaceous peripheral sealer on the side of the electrode parallel to the flow channels of the reactant gas therein has been joined to the compact and carbonaceous separator and the composed materials have been formed to be one body.
Still more, the sixth object of the present invention is to provide a composite electrode substrate for a fuel cell, whereithe compact and carbonaceous peripheral sealer on the side of the electrode parallel to the flow channels of the reactant gas therein and the compact and carbonaceous gas-distributor on the side of the electrode perpendicular to the flow channels of the reactant gas therein have been joined to the compact and carbonaceous separator and the composed materials have been formed to be one body.
In addition, the seventh object of the present invention is to provide a composite electrode substrate for a fuel cell, wherein the compact and carbonaceous manifold provided with a flow passage for supplying the reactant gas has been joined to the compact and carbonaceous separator and the composed materials have been formed to be one body.
Finally, the eighth object of the present invention is to provide a composite electrode substrate for a fuel cell of phosphoric acid type, which is excellent in resistance to phosphoric acid.
The other objects and the merits of the present invention will be clear to the persons skilled in the art from the following description of the present invention.